Whey, which is a byproduct of many commercial dairy processes, contains a large amount of lactose. The lactose in whey represents a large potential carbon and energy source, particularly for the production of ethanol. However, only a few commercial processes currently exist which utilize the lactose contained in whey, and converts it to a commercially useful product. Further, the existing processes are expensive, and, as a result, a great deal of whey is now disposed of, requiring costly waste treatment processes.
Various attempts to utilize the lactose contained in whey have included fermentation by strains of Kluyveromyces fragilis and other yeasts, especially Saccharomyces cerevisiae. The main problem with the use of S. cerevisiae is that it cannot ferment or utilize lactose directly. The lactose must first be hydrolyzed to form glucose and galactose, which the S. cerevisiae may then use. This procedure is inefficient since it produces high concentrations of extracellular glucose which cause catabolite repression of galactose utilization. Catabolite repression has been somewhat overcome by the selection of mutant strains that are resistant to repression. Despite these problems, S. cerevisiae is a desirable yeast to use in lactose utilizing processes since it has been used in the brewing and baking industries for many years, and procedures for its use on a commercial scale are highly developed. Further, S. cerevisiae may be genetically manipulated by various techniques, including genetic engineering. Finally, from the standpoint of basic research, it would be very advantageous to produce strains of S. cerevisiae that could grow on lactose because they could be used in a variety of mutant selection schemes, as has been accomplished with E. coli. It would therefore be advantageous to produce a strain of S. cerevisiae capable of direct lactose utilization and fermentation.
S. cerevisiae cannot use lactose because it lacks a beta-galactosidase structural gene and therefore cannot hydrolyze lactose to glucose and galactose. Further, it has no mechanism for transporting lactose across its cell membrane. This lack of a lactose transport mechanism has been demonstrated by direct measurement of lactose transport in S. cerevisiae and by a showing that genetically engineered strains of S. cerevisiae that produce an intercellular betagalactosidase also do not grow on lactose. One method of creating a transport mechanism in S. cerevisiae would be to introduce a lactose permease gene, such as the lac Y gene of E. coli, into a strain of S. cerevisiae that has been genetically engineered to produce beta-galactosidase. However, this approach, for unknown reasons, has never proven successful. An alternative approach would be to incorporate into the S. cerevisiae a lactose permease gene from another yeast.
The yeast Kluyveromyces lactis can grow on lactose as a sole carbon source, and is known to have an inducible lactose permease system. The genes coding for the permease have not yet been identified. Unsuccessful attempts have been made to isolate the lactose permease gene by transforming a K. lactis clone bank into a strain of S. cerevisiae that synthesizes beta-galactosidase and selecting transformants for growth on lactose.